The introduction of monofunctional aromatic chloroformates into a polymer synthesis provides a means to control the molecular weight of the polymer to be formed. In general, the greater the quantity of endcapping agent introduced into a polymer synthesis, the lower the molecular weight of the polymer product. Monofunctional aromatic chloroformates are particularly suitable as endcapping agents in interfacial polycarbonate synthesis because they enable production of a polycarbonate in a single step phosgenation with a substantially lower level of diarycarbonate(s) (DAC) than products produced using a hydroxyaromatic endcap, such as p-cumyl phenol.
Diarylcarbonates have a low melting point, compared with the glass transition temperature of polycarbonate, and are therefore the last components to freeze during a polycarbonate molding operation. Therefore, polycarbonate with significant levels of DAC requires longer molding cycle times compared with polycarbonate that is substantially free of DAC. Further, because DAC can sublime, a polycarbonate containing diaryl carbonates can lead to undesirable effects, such as "plate out" in which the DAC from previous molding cycles condenses 20 and deposits on the mold and leads to blemishes in subsequent moldings. The term "DAC" as used herein is understood to include also di(alkylphenyl carbonates) and di(arylphenyl)carbonates.
In making monofunctional aromatic chloroformates, it would be desirable to minimize production of by-product DAC. This would enable the monofunctional aromatic chloroformate to be used in a subsequent polymerization reaction without first being purified by such methods as distillation. In the following discussion, the term "MAC" or "MACs" refers to a monofunctional aromatic chloroformate compound or mixture of monofunctional aromatic chloroformate compounds.
Known process for the production of MACs by an interfacial process include the batchwise production of MACs, with subsequent storage for later use in polymerization.
U.S. Pat. No. 5,399,657 (Van Hout et al) discloses a method of preparing MAC in a batch process. A solution of phosgene in a solvent is introduced into a reactor, to which phosgene and a phenol compound are then added while maintaining the temperature at a value in the range of 3 to 5.degree. C. The pH is maintained within a desired range by addition of an aqueous caustic solution. Excess phosgene is then depleted from the product by reaction with caustic. The production of MACs in U.S. Pat. No. 5,399,657 involve long batch times, typically in the range of 30 to 60 minutes.
U.S. Pat. No. 5,274,164 (Wettling et al) discloses a method of preparing aryl chloroformates by the reaction of phenols with phosgene in the presence of organic phosphorous compounds. The process requires long reaction times, and the addition of a catalyst, such as the organic phosphorous compounds, necessitates extra process steps to recover the catalyst from the product.
U.S. Pat. No. 4,864,011 (Bussink et al) discloses a method of preparation of an aromatic polycarbonate with a MAC endcapping agent. According to Bussink, the MAC is either present prior to phosgene addition, or is added at a single point in the batch polymerization reaction to produce polycarbonate with low DAC. This process, however, has several disadvantages. In order to deliver MAC to the polymerization reaction at a specific point during the polymerization, the MAC must be synthesized, purified, and stored. Further, delivery of a quantity of MAC at a specific point in the batch process usually requires an additional apparatus for storage and charging.
It would be desirable to develop a process whereby the MACs may be produced directly, without the need for purification, and in a continuous manner. It would be even more desirable to develop a continuous process whereby the MACs could be produced in an "on-demand" manner. This would permit direct coupling of the MAC process to a batch or continuous polymerization process, particularly a polycarbonate synthesis process. Such a directly coupled process would be desirable because it would avoid the risks associated with maintaining an inventory of MAC and phosgene-containing materials associated with MAC production.
An on-demand process for MAC synthesis would further provide a significant reduction in both phosgene exposure risks and cost of production compared with a batch process for MAC synthesis. None of the disclosures discussed above meet these criteria.
It would further be desirable to develop a process requiring shorter processing times to produce the MACs that may be coupled with a continuous or batch process for polycarbonate synthesis, respectively (without purification of the MAC) to produce a product having low DAC content and good quality.
It would also be desirable to develop a process in which excellent molecular weight control of a polycarbonate produced in an interfacial reaction is achieved. Molecular weight control is usually measured by the standard deviation of the molecular weight for a series of batches. Good molecular weight control, i.e. control of the variability of molecular weight of the polycarbonate produced in a reaction or series of reaction, is directly related to control of the molecular weight viscosity. Molecular weight determines molecular weight viscosity; therefore maintaining the molecular weight in a narrow range results in the maintenance of molecular weight viscosity in a narrow range. It is desirable to maintain the molecular weight viscosity in a narrow range to control processibility of the product. For example, narrow control of the molecular weight viscosity over a series of product batches would enable a molding machine that is processing polycarbonate from these batches to operate for extended periods of time without adjustment.